MECHANISM OF THE NONSEQUENTIAL DOUBLE IONIZATION OF HELIUM

摘要

Model one-dimensional calculations show that there is a significant signature of nonsequential double ionization of He for all wavelengths, in the range from 248 to 1064 nm, provided that the laser pulse is sufficiently short. In this work, we show how the pulse duration can modify the "knee" structure of the double ionization yield curve. By using half-cycle pulses or by artificially decoupling the "irnner" electron from the laser, we show how one can test the validity of the proposed shake-off or recollision mechanisms for the double ionization. The flexibility of our model calculations allows us to probe the basic mechanism of the double ionization. The importance of the role of nonsequential (NS) ionization in multi-electron atomic systems subjected to an external intense laser field has been revealed in the last two decades in a series of experiments. The first observation of NS ionization in noble gases was made by L'Huillier et al. [1]. Fittinghoff et al. and Walker et al. [2] have made so far the most accurate measurements of the ion yields of Helium at infrared wavelengths. In both cases, the double-ionization signal is up to six orders of magnitude higher than expected from sequential ionization, and it saturates at the single ionization saturation intensity. For higher intensities it resumes its increase, but this time following the sequential electron ejection curve. This feature leads to the formation of a typical structure on the double ionization yield curve, known as a "knee", and it is evidence of the role played by the electron-electron interaction. More recent experiments have revealed that NS ionization is also present in diatomic molecules and in rare gas atoms at various laser wavelengths. The theoretical calculations that have been developed in the last few years in order to explain the very high degree of NS double ionization, are based on very different approaches. S-matrix theory by Becker and Faisal, "exact" fully correlated one-dimensional models and numerical models with partial correlation have all successfully reproduced the knee structure. Other attempts involving the full three-dimensional dynamics of the two correlated electrons have been made with supercomputers, but with only a very restricted range of intensities. In this work, we use one-dimensional models that have either full electron correlation, or treat the two electrons in a way that involves their mean fields in a partially correlated simple model that is also known as "crapola". For a review of some of the recent work regarding double ionization, see reference.